Certain embodiments of the present invention relate to ultrasound imaging of the human anatomy for the purpose of medical diagnosis. In particular, certain embodiments of the present invention relate to methods and apparatus for improving spatial and temporal resolution in the ultrasound image.
Two dimensional (2D) ultrasound Doppler imaging is used for blood flow visualization within the body, as well as for visualization of muscular tissue movement and deformation, such as forced compression of the human breast. A B-mode grayscale sector and 2D Doppler information in a sector covering all or part of the B-mode sector may be separately acquired. The Doppler information is color-coded and overlaid onto the B-mode grayscale image to visualize the velocity information of an area of interest. Because the grayscale image is used to visualize tissue structures and the Doppler image is used to represent velocity information, the B-mode image is often referred to as the xe2x80x9ctissue imagexe2x80x9d.
Prior techniques combine a high resolution 2D B-mode image with a lower resolution 2D Doppler image, acquired with the same frame rate. For example, FIG. 2 illustrates the geometry for a conventional sector scanned 2D Doppler acquisition. The geometry image 202 shows a Doppler sector image overlaid on a B-mode sector image. The B-mode sector image 204 is comprised of B-mode transmit beams 206. The Doppler sector image 208 is comprised of Doppler transmit beam directions 210. In this example, the number of B-mode transmit beams (NB) is 12, and the number of Doppler transmit beam directions (ND) is 4. Thus, the B-mode sector image 204 has a higher beam density and a higher resolution than the Doppler sector image 208.
FIG. 3 illustrates a scan sequence of a conventional 2D Doppler acquisition. Twelve Doppler pulses 302-324 and twelve B-mode pulses 326-348 are illustrated. First, the Doppler pulses 302-324 are transmitted sequentially, starting with Doppler pulse 302. Then the B-mode pulses 326-348 are transmitted sequentially, starting with B-mode pulse 326. The B-mode pulses 326-348 are labeled to indicate transmission direction. For example, B-mode pulse 326 labeled B1 indicates that the B-mode pulse is transmitted in direction 1. B-mode pulse 346 labeled B11 indicates that the pulse is transmitted in direction 11. The Doppler pulses 302-324 are labeled such that D indicates a Doppler pulse transmission j in direction i. The Doppler pulses 302-324 are each part of a packet 350-356. Each Doppler pulse 302-324 transmitted in the same direction is part of the same packet 350-356. The packet size (PS) is the number of Doppler pulses 302-324 in each packet 350-356. For example, the PS of FIG. 3 is 3. Therefore, each packet 350-356 comprises the Doppler pulses 302-324 transmitted in one transmit beam direction, and each packet 350-356 is transmitted in a different direction.
The velocities for 2D Doppler are estimated at points along each beam direction based on the received signals from the packets 350-356. For example, the Doppler pulses 302, 304, and 306 each may be used to estimate a velocity measurement for packet 350 in beam direction 1. The time between two Doppler pulses within a packet is called the Doppler pulse repetition time (PRTD), thus the Doppler pulse repetition frequency (PRFD) is PRFD=1/PRTD.
The depth in the body of the item of interest will determine the maximum PRFD (PRFDMAX). The transmitted Doppler pulse 302-324 must propagate to the deepest item of interest and back to the transducer before a new pulse transmission can be made. Additionally, hardware limitations and reverberations from deep reflectors beyond the imaging depth may need to be considered. For FIG. 3, PRFD greater than 0.5*PRFDMAX.
The time required to acquire one frame of Doppler and B-mode data is Tframe. The frame rate (FR) is calculated as FR=1/Tframe. Because one complete B-mode image frame is acquired between each Doppler image frame, the frame rate for Doppler (FRD) is equal to the frame rate for B-mode (FRB). During the acquisition of one image frame, the Doppler pulses 302-324 are each transmitted one time and the B-mode pulses 326-348 are each transmitted one time. The time to acquire one frame Tframe) may be calculated as:
Tframe=(NDxc3x97PS)/PRFD+NB/PRFBxe2x80x83xe2x80x83Equation 1
where Tframe is the time to acquire one image frame, ND is the number of Doppler transmit beam directions, PS is the packet size, PRFD is the Doppler pulse repetition frequency, NB is the number of B-mode transmit pulses per frame, and PRFB is the B-mode pulse repetition frequency. In FIG. 3, for example, ND=4, PS=3, and NB=12.
FIG. 4 illustrates a scan sequence of a conventional 2D Doppler acquisition utilizing interleaving of the Doppler pulses and the B-mode pulses. Twelve Doppler pulses 402-424 and twelve B-mode pulses 426-448 are illustrated. The Doppler pulses 402-424 are each part of a packet 450-456. Each packet 450-456 comprises the pulses transmitted in one beam direction, and each packet 450-456 is transmitted in a different direction.
As in FIG. 3, FIG. 4 has 4 Doppler transmit beam directions. In FIG. 4, however, the transmit beam directions, each comprised of three Doppler pulses 402-424, are interleaved with the B-mode pulses 426-448 . First, Doppler pulses 402-406 are transmitted in direction 1. Next, B-mode pulses 426-430 are transmitted, then Doppler pulses 408-412 are transmitted in direction 2, and so on. By interleaving the B-mode pulses 426-448 into the Doppler pulse 402-424 sequence, the timing difference between acquiring the Doppler image and acquiring the underlying B-mode image is reduced. The acquisition time per frame has not changed however, thus the time to acquire the Doppler scan image is the same as the time to acquire the B-mode scan image. To put it another way, one B-mode image is acquired for every Doppler image. Therefore, the FRD is equal to the FRB.
When lower velocities are measured, the PRFD may be decreased. If the PRFD decreases, the acquisition time per frame may increase and the frame rate may decrease, as illustrated by Equation 1. The frame rate may be maintained, however, by utilizing Doppler beam interleaving. After transmitting a Doppler pulse in a first direction, Doppler pulses are transmit in one or more other directions before transmitting the second pulse in the first direction. In Doppler beam interleaving, the Interleave Group Size (IGS) indicates the number of Doppler beam directions that are interleaved.
Therefore, for lower velocities, the same frame rate can be maintained with the same number of transmit directions by using Doppler beam interleaving where IGS is an integer xe2x89xa72, and PRFDxe2x89xa6PRFDmax/IGS. If PRFDmax=PRFD*IGS is kept constant, the scanning time per frame remains constant when PRFD is reduced. Thus, PRFDmax may be kept constant by increasing the IGS when the PRFD decreases, as illustrated in the following relationship:
Tframe=(NDxc3x97PS)/(PRFDxc3x97IGS)+NB/PRFB=(NDxc3x97PS)/PRFDmax+NB/PRFB
FIG. 5 illustrates a scan sequence of a conventional 2D Doppler acquisition with 2 Doppler transmit directions interleaved. Twelve Doppler pulses 502-524 followed in time by twelve B-mode pulses 526-548 are illustrated.
As described in FIG. 3, Doppler pulses are each part of a packet that comprises the pulse transmissions along one beam direction in the image. In FIG. 3, all of the Doppler pulses that comprise a packet are transmitted before transmitting a Doppler pulse of a different packet. In FIG. 5, however, the Doppler pulses 502-524 utilize Doppler beam interleaving as discussed previously. Doppler pulse 502 is transmitted in direction 1, then Doppler pulse 504 is transmitted in direction 2. Next, Doppler pulse 506 is transmitted in direction 1, then Doppler pulse 508 is transmitted in direction 2. The IGS of FIG. 5 is 2, because two Doppler transmit beam directions are interleaved. Once the Doppler transmit beams are completed, then the B-mode pulses are transmitted.
FIG. 6 illustrates a scan sequence of a conventional 2D Doppler acquisition with 4 Doppler transmit directions interleaved. Twelve Doppler pulses 602-624 and twelve B-mode pulses 626-648 are illustrated. One Doppler pulse 602-624 is transmitted in each of the 4 transmit directions before a second Doppler pulse 602-624 is transmitted in any direction. The IGS of FIG. 6 is 4.
In addition to the techniques above, it is possible to obtain several received beams for each transmitted pulse by focusing in slightly different directions. This technique is called parallel beamforming or Multi-Line Acquisition (MLA). The number of parallel receive beams per B-mode transmit beam (MLAB) may be different than the number of parallel receive beams per Doppler transmit beam (MLAD).
Below is an example of the frame rate and beam densities achieved with a conventional packet acquisition setup utilizing MLA for cardiac imaging. In this example, the PRFB is lower than the PRFD to minimize reverberation effects:
For tissue Doppler techniques there is a desire for frame rates considerably higher than what is achievable with the conventional 2D Doppler acquisition techniques discussed previously. Often the need to capture the details of flow jets or rapid tissue accelerations requires a high frame rate for the Doppler information, whereas the tissue B-mode image need not be updated as often. However, in order to achieve a high resolution B-mode image, the combined B-mode/Doppler frame rate becomes relatively low. For example, when assessing the rapid movement in the cardiac muscle during the relaxation phase of the cardiac cycle, the main problem with the aforementioned acquisition techniques is that the 2D Doppler frame rate can not be increased without decreasing the spatial resolution of the B-mode image. But in order to achieve the desired B-mode resolution in a sector covering the whole myocardium, the frame rate has to be reduced. For example, the frame rate of 55 Hz for conventional packet acquisition utilizing MLA as indicated above is much lower than the desired frame rate for cardiac imaging, which may be from 100 frames per second to as high as 300 frames per second for some applications.
Additionally, during B-mode imaging, there may be different demands on the resolution in different areas of a B-mode image. An example is when studying a heart valve. In a small region surrounding the valve, both high spatial and temporal resolution are desired. The other parts of the image are mainly used for orientation, and a lower resolution is acceptable.
Thus, a need has long existed in the industry for a method and apparatus for acquiring ultrasound data that addresses the problems noted above and previously experienced.
In accordance with at least one embodiment, a method is provided to simultaneously acquire two ultrasound images. A first set of ultrasound pulses is transmitted at a first frame rate in accordance with a first mode of operation. The echoes from the first set of ultrasound pulses are received. A second set of ultrasound pulses is transmitted at a second frame rate different from the first frame rate in accordance with a second mode of operation. The echoes from the first and second set of ultrasound pulses are displayed as one image.
In an alternative embodiment, the first set of ultrasound pulses defines a Doppler image and the second set of ultrasound pulses defines a portion of a B-mode image. A first portion of the B-mode image may be obtained before the Doppler image is obtained. Then a second portion of the B-mode image is obtained after the Doppler image is obtained. The Doppler and B-mode images are overlaid to display one image.
In another embodiment, the first set of ultrasound pulses defines a high resolution B-mode image and the second set of ultrasound pulses defines a low resolution B-mode image. The portion of the low resolution B-mode image underlying the high resolution B-mode image may be obtained using the ultrasound pulses defining the high resolution B-mode image. The high and low resolution B-mode images are overlaid to display one image.
In an alternative embodiment, a portion of a Doppler image may be calculated by transmitting a series of uninterrupted, successive pulses in a common direction and detecting the echoes returned from the series of successive pulses. A first packet of successive Doppler pulses directed in a first direction is transmitted, followed by a second packet of successive Doppler pulses directed in a second direction. In another embodiment, the Doppler pulses may be interleaved, wherein one pulse of the first packet is transmitted followed by one pulse of the second packet. In an alternative embodiment, the first set of ultrasound pulses and the second set of ultrasound pulses may be interleaved. The first packet of successive Doppler pulses is transmitted in a first direction. After the non-Doppler echoes are received, a second packet of successive Doppler pulses is transmitted in a second direction.
In an alternative embodiment, an image based on the received echoes from the Doppler pulses is comprised of a number of transmit directions, and one Doppler pulse is transmitted in each direction. A Doppler image is calculated by utilizing a sliding window technique based upon the received echoes.
In accordance with at least one embodiment, a method for obtaining ultrasound images of an area of interest is provided. A set of Doppler pulses is transmitted and the Doppler echoes are received. A set of non-Doppler pulses corresponding to a sub-region of a displayed image is transmitted and the non-Doppler echoes are received. Images based on the Doppler and non-Doppler echoes are displayed.
In one embodiment, the non-Doppler pulses correspond to a sub-region of an image. In another embodiment, the Doppler echoes form a complete image and the non-Doppler echoes form a partial image. Therefore, the set of Doppler pulses defines more image frames than the set of non-Doppler pulses.
In another embodiment, a scan sequence of transmitting and receiving pulses is divided into scan intervals. The scan interval in which non-Doppler pulses are transmitted and received may be suspended. In another embodiment, non-Doppler pulses associated with a first sub-region of a non-Doppler image are transmitted in a first scan interval and non-Doppler pulses associated with a second sub-region of a non-Doppler image are transmitted in a second scan interval.